Fusion of Preoperative MRI and Postoperative FD-CT for Direct Evaluation of Cochlear Implants : An Analysis at 1.5 T and 3 T.

This study was carried out to evaluate the diagnostic value of merging preoperative magnetic resonance imaging (MRI) with postoperative flat-panel computed tomography (FD-CT) and compare it to standard postoperative FD-CT for assessment of cochlear implant (CI) insertion. The T2-weighted (T2w) constructive interference in steady state (CISS) data sets of preoperative 1.5 T and 3 T MRI scans of CI patients with both regular and adverse implant spiralization were co-registered with the corresponding postoperative FD-CT data sets using defined anatomic landmarks. These merged FD-CT/MRI volumes (CMV) were compared to the corresponding postoperative FD-CT MPRs in consensus reading with respect to qualitative, i.e. scala tympani spiralization, scala vestibuli spiralization, scalar translocation and quantitative, i.e. distance of the last electrode to the lateral cochlea wall (D1) distance of the 2nd/5th electrode to the basal cochlear wall (D2) and the transition point (TP) of the scalar translocation, parameters. In total 30 patients (n 1.5T MRI = 18 patients; n 3T MRI = 12 patients) were included in the analysis. In all cases both CMVs and FD-CT MPRs were generated. Qualitative analysis of intracochlear CI position with CMVs (both 1.5 T and 3 T) and FD-CT was equivalent: In 20 patients the CI showed a regular implant spiralization, in 10 cases a scalar translocation was identified with both CMVs and FD-CT. Quantitative analysis showed a high level of congruency between CMVs (both 1.5 T and 3T) and FD-CT for fusion accuracy (D1: mean FD-CT D1 = 1.30 ± 0.7 mm; mean CMV D1 = 1.27 ± 0.77 mm, correlation r = 0.94, p < 0.0001; D2: mean FD-CT D2 = 1.17 ± 0.34 mm; mean CMV D2 = 1.10 ± 0.31 mm, correlation r = 0.89, p < 0.0001) and TP of the scalar translocation (mean FD-CT = 126.0 ± 59.25°, mean CMV = 117.0 ± 52.82°, correlation r = 0.95, p < 0.0001). The co-registration of preoperative 1.5 and 3 T MRI with postoperative FD-CT enables a direct evaluation of the position of a CI equivalent to the current standard FD-CT. Despite the fact that CMV provided no additional diagnostic value in this series, regardless whether preoperative 1.5 or 3 T MRI was used for co-registration, it might help to simplify postoperative CI diagnostics.